Infertility is a major health care concern affecting millions of couples worldwide. Contributing to this problem, early demise of the human conceptus is a common event. Approximately 73% of natural single conceptions are lost before reaching week 6 of gestation (Boklage C E. Survival probability of human conceptions from fertilization to term. Int J Fertil 1990; 35:75). This is mostly due to early embryonic demise prior to implantation or soon after implantation occurs. Data relating to the low fertility rate observed in older women and its improvement by oocyte donation from young women indicate that oocyte quality is an important factor in achieving a successful pregnancy (Navot D, Bergh P A, Williams M A et al. Poor oocyte quality rather than implantation failure as a cause of age-related decline female fertility. Lancet 1991; 337:1375).
In vitro fertilization (“IVF”) is a technology which has been developed to address the problem of infertility. However, maintaining embryo viability is even more problematic under the artificial conditions used for culturing embryos in vitro for implantation. In vitro, the embryo development rate is lower than in vivo and only 25-65% of embryos typically develop to the blastocyst stage (Gardner D K, Lane M, KOuridakis K, Schoolvcraft W B. Complex physiologically based serum-free culture media increase mammalian embryo development. In:Gomel V, Leung P C K, eds. In vitro fertilization and assisted reproduction. Procc 10th World Congress, 1997:187). The state of the art is not yet able to identifying embryos likely to implant and survive. Human chorionic gonadotrophin (“hCG”), the currently used marker for fertilization in vivo and early embryo implantation, can only be detected several days after implantation. As a result of the lack of a suitable marker for embryo viability, nowadays many embryos incapable of implanting are being transferred, thus lowering the chance for achieving successful pregnancy.
To address the possibility that embryos may not be viable, a greater number of embryos are simultaneously transferred into a potential mother. The transfer of a high number of embryos may lead to multiple pregnancies, which are inherently risky, while transfer of a small number of embryos carries the risk that none would implant, losing a whole IVF cycle. Clearly, there is a need to improve embryo selection and define accurate markers to determine embryo viability. In addition, using non-invasive methods by testing culture media for products specific to viable, implantation-competent embryos would allow selection of those most likely to result in successful pregnancies, without causing embryo damage.
Another factor involved in determining whether a pregnancy is successful or not is the interaction between the conceptus and the mother's immune system. Shortly after fertilization a systemic maternal recognition of pregnancy should occur. The mother's immune system modulation triggered by specific early embryo signals could be the key of this process. Once the oocyte is fertilized, the zygote up to hatching blastocyst is surrounded by the zona pellucida, a hard semi permeable membrane. Therefore the embryo-maternal communication must occur simultaneously while the embryo is developing in the oviduct and uterine cavity through compounds that are secreted by the embryo.
It has been shown that pregnant sera and viable embryo conditioned culture media can produce an increase in rosette formation by platelets and T lymphocytes in the presence of CD2 antibody. As disclosed in U.S. Pat. No. 5,646,003 by Barnea et al., issued Jul. 8, 1997, and in U.S. Pat. No. 5,981,198 by Bamea et al., granted Nov. 9, 1999, the presence of Preimplantation Factor (“PIF”) can be detected by mixing lymphocytes, platelets, heat inactivated serum from a pregnant subject, guinea pig complement, and T11 (anti-CD2) monoclonal antibody (Dakko, Denmark), where rosette formation between platelets and lymphocytes is increased by PIF in pregnant subjects. PIF has been found to be (i) secreted by viable early human and mouse embryos from the two-cell stage onward; detectable in the peripheral circulation 34 days after embryo transfer following IVF; (iii) associated with 73% take home babies vs 3% in early negative PIF results; (iv) detectable 5-6 days after intrauterine insemination; (v) absent in non-pregnant serum, or non-viable embryos; and (vi) present in various pregnant mammals in addition to humans, including mice, horses, cows and pigs. In addition, PIF has been observed to disappear from the circulation two weeks before hCG secretion declines in cases of spontaneous abortion.
The monoclonal antibody used in the above-mentioned PIF assay is directed toward the lymphocyte associated antigen referred to as CD2. CD2 is present on about 80-90% of human peripheral blood lymphocytes, greater than 95% of thymocytes, all T lymphocytes that form erythrocyte rosettes and a subset of NK cells. Various roles for CD2 in T cell activation have been proposed, including function as an adhesion molecule which reduces the amount of antigen required for T cell activation and as a costimulatory molecule or direct promoter of T cell activation. Moreover, CD2 has been implicated in the induction of anergy, the modulation of cytokine production and the regulation of positive selection of T-cells.
The natural ligand for CD2 is the structurally related IgSF CAMs CD58 (LFA-3), a cell-surface adhesive ligand with broad tissue distribution. In addition, CD2 can interact with CD48, CD59 and CD15 (Lewis x)-associated carbohydrate structure. CD2 binds CD58 with very low affinity and an extremely fast dissociation constant. The lateral redistribution of CD2 and its ligand CD58 also affect cellular adhesion strength. Regulation of CD2 adhesiveness affects the ability of CD2 to enhance antigen responsiveness. CD2-cell lines incapable of avidity regulation exhibit a marked deficiency in an antigen-specific response. Strength of adhesion resulting from increased CD2 avidity contributes directly to T-cell responsiveness independently of CD2-mediated signal transduction.